Chapter 2
BMP Selection
Contents
1.0
BMP Selection ................................................................................................................................... 1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
Physical Site Characteristics ...................................................................................................................... 1
Space Constraints ...................................................................................................................................... 2
Targeted Pollutants and BMP Processes ................................................................................................... 2
Storage-Based Versus Conveyance-Based ................................................................................................ 8
Volume Reduction ..................................................................................................................................... 8
Pretreatment ............................................................................................................................................... 9
Treatment Train ......................................................................................................................................... 9
Online Versus Offline Facility Locations ................................................................................................ 10
Integration with Flood Control ................................................................................................................ 10
1.9.1 Sedimentation BMPs ................................................................................................................... 11
1.9.2 Infiltration/Filtration BMPs ......................................................................................................... 11
1.10 Land Use, Compatibility with Surroundings, and Safety ........................................................................ 11
1.11 Maintenance and Sustainability ............................................................................................................... 12
1.12 Costs ........................................................................................................................................................ 12
2.0
3.0
BMP Selection Tool ........................................................................................................................ 13
Life Cycle Cost and BMP Performance Tool ............................................................................... 16
3.1 BMP Whole Life Costs............................................................................................................................ 16
3.2 BMP Performance ................................................................................................................................... 17
3.3 Cost Effectiveness ................................................................................................................................... 17
4.0
Conclusion ....................................................................................................................................... 17
5.0
References ....................................................................................................................................... 18
Figures
Figure 2-1. BMP Decision Tree for Highly Urbanized Sites ..................................................................... 13
Figure 2-2. BMP Decision Tree for Conventional Development Sites...................................................... 14
Figure 2-3. BMP Decision Tree for Linear Construction in Urbanized Areas .......................................... 15
Tables
Table 2-1. Primary, Secondary and Incidental Treatment Process Provided by BMPs ............................... 5
Table 2-2. BMP Effluent EMCs (Source: International Stormwater BMP Database, August 2010) ........ 6
August 2011
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Chapter 2
1.0
BMP Selection
BMP Selection
This chapter provides guidance on factors that should be considered when selecting BMPs for new
development or redevelopment projects. This guidance is particularly useful in the planning phase of a
project. BMP selection involves many factors such as physical site characteristics, treatment objectives,
aesthetics, safety, maintenance requirements, and cost. Typically, there is not a single answer to the
question of which BMP (or BMPs) should be selected for a site; there are usually multiple solutions
ranging from stand alone BMPs to treatment trains that combine multiple BMPs to achieve the water
quality objectives. Factors that should be considered when selecting BMPs are the focus of this chapter.
1.1
Physical Site Characteristics
The first step in BMP selection is identification of physical characteristics of a site including topography,
soils, contributing drainage area, groundwater, baseflows, wetlands, existing drainageways, and
development conditions in the tributary watershed (e.g., construction activity). A fundamental concept of
Low Impact Development (LID) is preservation and protection of site features including wetlands,
drainageways, soils that are conducive to infiltration, tree canopy, etc., that provide water quality and
other benefits. LID stormwater treatment systems are also designed to take advantage of these natural
resources. For example, if a portion of a site is known to have soils with high permeability, this area may
be well-suited for rain gardens or permeable pavement. Areas of existing wetlands, which would be
difficult to develop from a Section 404 permitting perspective, could be considered for polishing of runoff
following BMP treatment, providing additional water quality treatment for the site, while at the same time
enhancing the existing wetlands with additional water supply in the form of treated runoff.
Some physical site characteristics that provide opportunities for BMPs or constrain BMP selection
include:

Soils: Soils with good permeability, most typically associated with Hydrologic Soil Groups (HSGs)
A and B provide opportunities for infiltration of runoff and are well-suited for infiltration-based
BMPs such as rain gardens, permeable pavement systems, sand filter, grass swales, and buffers, often
without the need for an underdrain system. Even when soil permeability is low, these types of BMPs
may be feasible if soils are amended to increase permeability or if an underdrain system is used. In
some cases, however, soils restrict the use of infiltration based BMPs. When soils with moderate to
high swell potential are present, infiltration should be avoided to minimize damage to adjacent
structures due to water-induced swelling. In some cases, infiltration based designs can still be used if
an impermeable liner and underdrain system are included in the design; however, when the risk of
damage to adjacent infrastructure is high, infiltration based BMPs may not be appropriate. In all
cases, consult with a geotechnical engineer when designing infiltration BMPs near structures.
Consultation with a geotechnical engineer is necessary for evaluating the suitability of soils for
different BMP types and establishing minimum distances between infiltration BMPs and structures.

Watershed Size: The contributing drainage area is an important consideration both on the site level
and at the regional level. On the site level, there is a practical minimum size for certain BMPs,
largely related to the ability to drain the WQCV over the required drain time. For example, it is
technically possible to size the WQCV for an extended detention basin for a half-acre site; however,
designing a functional outlet to release the WQCV over a 40-hour drain time is practically impossible
due to the very small orifices that would be required. For this size watershed, a filtering BMP, such
as a rain garden, would be more appropriate. At the other end of the spectrum, there must be a limit
on the maximum drainage area for a regional facility to assure adequate treatment of rainfall events
that may produce runoff from only a portion of the area draining to the BMP. If the overall drainage
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Chapter 2
area is too large, events that produce runoff from only a portion of the contributing area will pass
through the BMP outlet (sized for the full drainage area) without adequate residence time in the BMP.
As a practical limit, the maximum drainage area contributing to a water quality facility should be no
larger than one square mile.

Groundwater: Shallow groundwater on a site presents challenges for BMPs that rely on infiltration
and for BMPs that are intended to be dry between storm events. Shallow groundwater may limit the
ability to infiltrate runoff or result in unwanted groundwater storage in areas intended for storage of
the WQCV (e.g., porous sub-base of a permeable pavement system or in the bottom of an otherwise
dry facility such as an extended detention basin). Conversely, for some types of BMPs such as
wetland channels or constructed wetland basins, groundwater can be beneficial by providing
saturation of the root zone and/or a source of baseflow. Groundwater quality protection is an issue
that should be considered for infiltration-based BMPs. Infiltration BMPs may not be appropriate for
land uses that involve storage or use of materials that have the potential to contaminate groundwater
underlying a site (i.e., "hot spot" runoff from fueling stations, materials storage areas, etc.). If
groundwater or soil contamination exists on a site and it will not be remediated or removed as a part
of construction, it may be necessary to avoid infiltration-based BMPs or use a durable liner to prevent
infiltration into contaminated areas.

Base Flows: Base flows are necessary for the success of some BMPs such as constructed wetland
ponds, retention ponds and wetland channels. Without baseflows, these BMPs will become dry and
unable to support wetland vegetation. For these BMPs, a hydrologic budget should be evaluated.
Water rights are also required for these types of BMPs in Colorado.

Watershed Development Activities (or otherwise erosive conditions): When development in the
watershed is phased or when erosive conditions such as steep slopes, sparse vegetation, and sandy
soils exist in the watershed, a treatment train approach may be appropriate. BMPs that utilize
filtration should follow other measures to collect sediment loads (e.g., a forebay). For phased
developments, these measures must be in place until the watershed is completely stabilized. When
naturally erosive conditions exist in the watershed, these measures should be permanent. The
designer should consider existing, interim and future conditions to select the most appropriate BMPs.
1.2
Space Constraints
Space constraints are frequently cited as feasibility issues for BMPs, especially for high-density, lot-lineto-lot-line development and redevelopment sites. In some cases, constraints due to space limitations arise
because adequate spaces for BMPs are not considered early enough in the planning process. This is most
common when a site plan for roads, structures, etc., is developed and BMPs are squeezed into the
remaining spaces. The most effective and integrated BMP designs begin by determining areas of a site
that are best suited for BMPs (e.g., natural low areas, areas with well-drained soils) and then designing
the layout of roads, buildings, and other site features around the existing drainage and water quality
resources of the site. Allocating a small amount of land to water quality infrastructure during early
planning stages will result in better integration of water quality facilities with other site features.
1.3
Targeted Pollutants and BMP Processes
BMPs have the ability to remove pollutants from runoff through a variety of physical, chemical and
biological processes. The processes associated with a BMP dictate which pollutants the BMP will be
effective at controlling. Primary processes include peak attenuation, sedimentation, filtration, straining,
adsorption/absorption, biological uptake and hydrologic processes including infiltration and
evapotranspiration. Table 2-1 lists processes that are associated with BMPs in this manual. For many
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BMP Selection
sites, a primary goal of BMPs is to remove gross solids, suspended sediment and associated particulate
fractions of pollutants from runoff. Processes including straining, sedimentation, and infiltration/filtration
are effective for addressing these pollutants. When dissolved pollutants are targeted, other processes
including adsorption/absorption and biological uptake are necessary. These processes are generally
sensitive to media composition and contact time, oxidation/reduction potential, pH and other factors. In
addition to pollutant removal capabilities, many BMPs offer channel stability benefits in the form of
reduced runoff volume and/or reduced peak flow rates for frequently occurring events. Brief descriptions
of several key processes, generally categorized according to hydrologic and pollutant removal functions
are listed below:
Hydrologic Processes
1. Flow Attenuation: BMPs that capture and slowly release the WQCV help to reduce peak discharges.
In addition to slowing runoff, volume reduction may also be provided to varying extents in BMPs
providing the WQCV.
2. Infiltration: BMPs that infiltrate runoff reduce both runoff peaks and surface runoff volumes. The
extent to which runoff volumes are reduced depends on a variety of factors such as whether the BMP
is equipped with an underdrain and the characteristics and long-term condition of the infiltrating
media. Examples of infiltrating BMPs include (unlined) sand filters, bioretention and permeable
pavements. Water quality treatment processes associated with infiltration can include filtration and
sorption.
3. Evapotranspiration: Runoff volumes can be reduced through the combined effects of evaporation
and transpiration in vegetated BMPs. Plants extract water from soils in the root zone and transpire it
to the atmosphere. Evapotranspiration is the hydrologic process provided by vegetated BMPs,
whereas biological uptake may help to reduce pollutants in runoff.
Pollutant Removal/Treatment Processes
1. Sedimentation: Gravitational separation of particulates from urban runoff, or sedimentation, is a key
treatment process by BMPs that capture and slowly release runoff. Settling velocities are a function
of characteristics such as particle size, shape, density, fluid density, and viscosity. Smaller particles
under 60 microns in size (fine silts and clays) (Stahre and Urbonas, 1990) can account for
approximately 80% of the metals in stormwater attached or adsorbed along with other contaminants
and can require long periods of time to settle out of suspension. Extended detention allows smaller
particles to agglomerate into larger ones (Randall et al, 1982), and for some of the dissolved and
liquid state pollutants to adsorb to suspended particles, thus removing a larger proportion of them
through sedimentation. Sedimentation is the primary pollutant removal mechanism for many
treatment BMPs including extended detention basins, retention ponds, and constructed wetland
basins.
2. Straining: Straining is physical removal or retention of particulates from runoff as it passes through
a BMP. For example, grass swales and grass buffers provide straining of sediment and coarse solids
in runoff. Straining can be characterized as coarse filtration.
3. Filtration: Filtration removes particles as water flows through media (often sand or engineered
soils). A wide variety of physical and chemical mechanisms may occur along with filtration,
depending on the filter media. Metcalf and Eddy (2003) describe processes associated with filtration
as including straining, sedimentation, impaction, interception, adhesion, flocculation, chemical
adsorption, physical adsorption, and biological growth. Filtration is a primary treatment process
August 2011
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Chapter 2
provided by infiltration BMPs. Particulates are removed at the ground surface and upper soil horizon
by filtration, while soluble constituents can be absorbed into the soil, at least in part, as the runoff
infiltrates into the ground. Site-specific soil characteristics, such as permeability, cation exchange
potential, and depth to groundwater or bedrock are important characteristics to consider for filtration
(and infiltration) BMPs. Examples of filtering BMPs include sand filters, bioretention, and
permeable pavements with a sand filter layer.
4. Adsorption/Absorption: In the context of BMPs, sorption processes describe the interaction of
waterborne constituents with surrounding materials (e.g., soil, water). Absorption is the incorporation
of a substance in one state into another of a different state (e.g., liquids being absorbed by a solid).
Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another
molecule. Many factors such as pH, temperature and ionic state affect the chemical equilibrium in
BMPs and the extent to which these processes provide pollutant removal. Sorption processes often
play primary roles in BMPs such as constructed wetland basins, retention ponds, and bioretention
systems. Opportunities may exist to optimize performance of BMPs through the use of engineered
media or chemical addition to enhance sorption processes.
5. Biological Uptake: Biological uptake and storage processes include the assimilation of organic and
inorganic constituents by plants and microbes. Plants and microbes require soluble and dissolved
constituents such as nutrients and minerals for growth. These constituents are ingested or taken up
from the water column or growing medium (soil) and concentrated through bacterial action,
phytoplankton growth, and other biochemical processes. In some instances, plants can be harvested
to remove the constituents permanently. In addition, certain biological activities can reduce toxicity
of some pollutants and/or possible adverse effects on higher aquatic species. Unfortunately, not much
is understood yet about how biological uptake or activity interacts with stormwater during the
relatively brief periods it is in contact with the biological media in most BMPs, with the possible
exception of retention ponds between storm events (Hartigan, 1989). Bioretention, constructed
wetlands, and retention ponds are all examples of BMPs that provide biological uptake.
When selecting BMPs, it is important to have realistic expectations of effluent pollutant concentrations.
The International Stormwater BMP Database (www.bmpdatabase.org) provides BMP performance
information that is updated periodically and summarized in Table 2-2. BMPs also provide varying
degrees of volume reduction benefits. Both pollutant concentration reduction and volume reduction are
key components in the whole life cycle cost tool BMP-REALCOST.xls (Roesner and Olson 2009)
discussed later in this chapter.
It is critical to recognize that for BMPs to function effectively, meet performance expectations, and
provide for public safety, BMPs must:
1. Be designed according to UDFCD criteria, taking into account site-specific conditions (e.g., high
groundwater, expansive clays and long-term availability of water).
2. Be constructed as designed. This is important for all BMPs, but appears to be particularly critical for
permeable pavements, rain gardens and infiltration-oriented facilities.
3. Be properly maintained to function as designed. Although all BMPs require maintenance,
infiltration-oriented facilities are particularly susceptible to clogging without proper maintenance.
Underground facilities can be vulnerable to maintenance neglect because maintenance needs are not
evident from the surface without special tools and procedures for access. Maintenance is not only
essential for proper functioning, but also for aesthetic and safety reasons. Inspection of facilities is an
important step to identify and plan for needed maintenance.
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Chapter 2
BMP Selection
Table 2-1. Primary, Secondary and Incidental Treatment Process Provided by BMPs
Hydrologic Processes
Peak
Volume
UDFCD BMP
Treatment Processes
Chemical
Physical
Flow
EvapoAdsorption/ Biological
Infiltration
Sedimentation Filtration Straining
Attenuation
transpiration
Absorption Uptake
Grass Swale
Grass Buffer
Constructed
Wetland Channel
Green Roof
Permeable
Pavement Systems
Bioretention
Extended
Detention Basin
Sand Filter
Constructed
Wetland Pond
Retention Pond
I
S
I
S
S
P
S
S
I
S
I
S
S
P
S
S
I
N/A
P
P
S
P
S
P
P
S
P
N/A
P
N/A
I
P
P
P
N/A
S
P
N/A
N/A
N/A
P
P
S
P
P
S
S1
P
P
I
I
P
N/A
S
S
I
P
P
I
P
P
N/A
S1
N/A
P
I
P
P
S
S
P
P
N/A
N/A
P
S
Variable
N/A
P
I
P
P
Underground
Variable
N/A
N/A
Variable
BMPs
Notes:
P = Primary; S = Secondary, I = Incidental; N/A = Not Applicable
1
Biological
Variable Variable
Depending on media
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Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Sample Type
88.0
22.3
79.5
42.0
(0.90-1.33, n = 7)
1.05
NC
29.1
(16.3-34.0, n = 7)
(1.80-3.30, n = 3)
2.40
(0.90-1.14, n= 8)
1.00
(0.77-1.30, n= 4)
1.10
(0.76-1.29, n= 30)
0.99
(0.77-1.42, n= 28)
1.18
(0.50-1.22, n= 17)
0.80
(0.87-2.00, n= 17)
1.53
(0.86-1.95, n= 10)
1.66
(0.77-1.70, n= 10)
1.32
(0.41-1.48, n= 16)
1.23
(1.40-2.11, n= 12)
1.83
(0.95-1.50, n= 13)
1.20
(1.15-2.10, n= 13)
1.40
(0.60-2.09, n= 8)
0.94
(1.00-1.33, n= 8)
1.22
Total Kjeldahl
Nitrogen (TKN)
(16.0-45.3, n = 5)
NA
NC
NA
23.5
1.16
(0.98-1.39, n= 6)
NC
(8.5-17.5, n= 16)
12.0
1.54
(1.07-2.16, n= 6)
NA
39.6
(1.01-1.54, n= 19)
1.31
(1.07-2.36, n= 19)
1.71
(0.43-1.41, n= 4)
0.63
(0.73-1.80, n= 5)
1.51
(1.7-2.69, n= 3)
2.54
(1.04-1.25, n= 3)
1.05
(0.55-1.34, n= 6)
0.60
NC
NC
(24.0-56.8, n= 14)
151.3
(70.8-182.0, n= 9)
12.1
(7.9-19.7, n= 40)
89.0
(59.3-127.5, n= 9)
44.5
(24.0-88.3, n= 40)
55.0
(46.0-62.0, n= 13)
8.0
(5.0-17.0, n= 21)
(28.4-59.0, n= 13)
44.0
(32.0-75.0, n= 21)
85.0
(54.3-113.5, n= 6)
22.0
(11.6-28.5, n= 20)
88.5
(85.0-98.8, n= 6)
59.5
(17.8-83.8, n= 18)
71.0
(34.9-85.0, n= 10)
18.0
(8.9-39.5, n= 19)
(30.5-76.5, n= 15) (64.2-100.1, n= 12)
54.5
(15.0-28.3, n= 14) (73.3-110.0, n= 12)
57.5
(32.0-89.3, n= 12)
52.3
(50.0-63.3, n= 14)
NC
1.15
(0.92-2.98, n= 7)
(6.8-17.3, n= 6)
12.9
1.46
Nitrogen,
Total
(1.24-1.63, n= 7)
NC
NC
Total
Dissolved
Solids
(41.8-53.3, n= 6)
44.6
Total
Suspended
Solids
NC
NC
(0.04-0.10, n= 8)
0.06
(0.04-0.13, n= 8)
0.10
(0.04-0.17, n= 24)
0.07
(0.04-0.15, n= 23)
0.09
(0.04-0.15, n= 10)
0.11
(0.08-1.12, n= 11)
0.34
(0.07-0.10, n= 5)
0.09
(0.04-0.10, n= 5)
0.08
(0.03-0.06, n= 8)
0.05
(0.02-0.09, n= 4)
0.06
(0.13-0.36, n= 9)
0.25
(0.23-0.64, n= 10)
0.38
(0.05-0.38, n= 8)
0.06
(0.16-0.23, n= 8)
0.19
Nitrogen,
Ammonia as N
NC
NC
(0.10-0.16, n= 7)
0.12
(0.32-0.44, n= 5)
0.32
(0.13-0.26, n= 15)
0.19
(0.32-0.69, n= 15)
0.43
(0.39-0.73, n= 16)
0.66
(0.23-0.57, n= 16)
0.38
(0.27-0.85, n= 8)
0.40
(0.30-0.90, n= 8)
0.45
(0.21-0.66, n= 15)
0.29
(0.23-0.78, n= 12)
0.41
(0.23-0.78, n= 13)
0.33
(0.42-0.92, n= 13)
0.44
NC
NC
Nitrogen, Nitrate
(NO3) as N*
(1.21-1.39, n = 4)
1.24
(0.27-0.80, n = 5)
0.59
(0.05-0.34, n= 7)
0.17
(0.11-0.63, n= 7)
0.46
(0.02-0.20, n= 24)
0.05
(0.11-0.55, n= 24)
0.27
(0.05-1.00, n= 5)
0.43
(0.23-0.51, n= 6)
0.33
(0.08-0.43, n= 6)
0.17
(0.17-0.50, n= 5)
0.23
(0.18-0.31, n= 8)
0.22
(0.19-0.37, n= 4)
0.25
NC
NC
(0.14-0.29, n= 10)
0.21
(0.25-0.38, n= 10)
0.30
Nitrogen, Nitrite
(NO2) + Nitrate
(NO3) as N*
(0.10-0.19, n = 5)
0.13
(0.10-0.13, n = 5)
0.12
(0.05-0.14, n= 13)
0.08
(0.14-0.27, n= 11)
0.12
(0.07-0.19, n= 40)
0.11
(0.14-0.39, n= 38)
0.23
(0.06-0.15, n= 21)
0.11
(0.13-0.33, n= 21)
0.20
(0.13-0.26, n= 18)
0.20
(0.18-0.30, n= 17)
0.20
(0.19-0.31, n= 19)
0.23
(0.13-0.29, n= 15)
0.22
(0.11-0.56, n= 14)
0.30
(0.09-0.25, n= 14)
0.18
(0.08-0.19, n= 12)
0.13
(0.12-0.17, n= 12)
0.13
Phosphorus as
P, Total
Table Notes provided below part 2 of this table.
*Some BMP studies include analyses for both NO2/NO3 and NO3; therefore, these analytes are reported separately, even though results are expected to be comparable in stormwater runoff.
Permeable
Pavement**
Wetland Basin
Retention Pond
(aboveground
wet pond)
Media Filters
(various types)
Detention Basin
(aboveground
extended det.)
Grass Swale
Grass Buffer
Bioretention
(w/Underdrain)
BMP Category
Solids and Nutrients (milligrams/liter)
Table 2-2. BMP Effluent EMCs (Source: International Stormwater BMP Database, August
NC
NC
(0.02-0.25, n= 7)
0.06
(0.07-0.13, n= 5)
0.04
(0.02-0.08, n= 27)
0.05
(0.07-0.21, n= 26)
0.09
(0.02-0.06, n= 7)
0.02
(0.02-0.06, n= 7)
0.02
NC
NC
(0.08-0.12, n= 7)
0.10
(0.03-0.04, n= 3)
0.04
(0.05-0.29, n= 10)
0.10
(0.03-0.06, n= 10)
0.04
(0.03-0.33, n= 7)
0.06
(0.01-0.10, n= 7)
0.04
Phosphorus,
Orthophosphate as
P
BMP Selection
Chapter 2
August 2011
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Urban Storm Drainage Criteria Manual Volume 3
Analyte
52.3
(50-63.3, n= 14)
22.3
(15-28.3, n= 14)
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Outflow
Inflow
Sample Type
0.2
0.2
(0.2-0.4, n= 3)
(1.0-1.4, n= 3)
1.0
(0.8-1.0, n= 3)
Description
= Median inflow value
= Interquartile range, sample size
= Median outflow value
= Interquartile range, sample size
NC
NC
NA
NA
NA
NA
NA
NA
(0.2-0.4, n= 3)
1.0
NC
0.2
(0.7-1.6, n= 12)
NC
NC
NC
NC
NC
NA
NA
(1.0-10.0, n= 4)
5.5
(1.6-10.0, n= 4)
5.9
(1.0-1.0, n= 13)
1.0
(1.0-1.0, n= 13)
1.0
(1.7-3.0, n= 4)
1.9
(2.0-3.2, n= 3)
2.6
(1.0-3.0, n= 6)
1.1
(1.1-3.3, n= 7)
2.2
(1.0-3.8, n= 12)
2.3
(1.1-4.5, n= 12)
2.4
NC
NC
Chromium,
Diss.
NC
NC
NA
NA
(1.4-5.3, n= 12)
2.2
(3.0-7.4, n= 12)
5.0
(1.0-1.9, n= 13)
1.0
(1.4-4.0, n= 13)
2.1
(1.9-3.8, n= 6)
2.9
(5.0-6.5, n= 6)
5.6
(1.7-5.0, n= 6)
3.5
(3.6-8.3, n= 7)
6.1
(2.0-5.5, n= 13)
2.9
(2.9-7.4, n= 13)
4.9
NC
NC
(4.5-6.4, n =4)
6.2
(2.5-6.4, n =3)
5.0
(5.0-5.7, n= 3)
5.0
NC
(4.7-5.8, n= 7)
5.0
(6.0-9.5, n= 7)
7.0
(3.1-8.3, n= 13)
5.8
(5.4-7.4, n= 13)
6.2
(3.0-13.0, n= 9)
9.0
(2.6-11.8, n= 8)
5.8
(5.5-9.7, n= 13)
8.6
(8.1-15.0, n= 13)
10.6
(4.8-11.6, n= 12)
7.1
(6.8-17.3, n= 12)
12.9
NC
NC
Metals (micrograms/liter)
Chromium,
Copper,
Total
Diss.
(3.0-14.7, n = 5)
9.0
(4.5-19.4, n = 3)
7.0
(3.3-5.0, n= 6)
4.5
(4.3-15.9, n= 4)
10.5
(3.0-6.2, n= 26)
5.4
(4.3-10.6, n= 26)
6.3
(4.3-9.6, n= 18)
7.3
(8.8-16.4, n= 18)
13.5
(6.2-20.1, n= 12)
11.0
(4.8-33.5, n= 11)
10.0
(6.7-18.5, n= 17)
14.0
(26-34, n= 13)
33.0
(6.4-12.5, n= 13)
8.3
(15.0-41.0, n= 13)
21.2
(7.3-16.8, n= 3)
10.0
(15.3-35.8, n= 3)
19.5
Copper,
Total
(0.04-0.5, n = 4)
0.3
(0.03-0.3, n = 3)
0.1
(0.8-1.0, n= 3)
1.0
NC
(1.0-4.9, n= 12)
1.2
(1.0-5.1.0, n= 11)
2.0
(1.0-1.0, n= 13)
1.0
(1.0-2.0, n= 14)
1.1
(0.5-1.3, n= 9)
1.0
(0.5-1.4, n= 8)
1.0
(0.5-4.1, n= 13)
1.0
(0.6-6.7, n= 13)
1.4
(0.5-1.3, n= 12)
0.5
(0.5-2.0, n= 12)
0.9
NC
NC
Lead,
Diss.
(1.3-9.5, n = 7)
2.5
(1.3-15.1, n = 3)
2.5
(1.0-2.5, n= 6)
1.0
(4.0-23.8, n= 4)
16.0
(1.6-10.0, n= 33)
4.7
(4-28, n= 33)
9.7
(1.0-4.4, n= 17)
1.6
(5.3-22.0, n= 17)
9.0
(1.3-18.6, n= 12)
9.5
(1.5-41.0, n= 11)
10.0
(1.7-12.0, n= 18)
10.5
(12.5-46.4, n= 14)
21.6
(1.8-6.0, n= 13)
3.2
(6-35, n= 13)
11.0
NC
NC
Lead,
Total
NC
NC
NA
NA
(7.2-10.0, n= 3)
10.0
(6.2-10.0, n= 3)
10.0
(2.0-2.6, n= 13)
2.0
(2.0-2.7, n= 13)
2.0
(2.0-3.2, n= 5)
3.1
(1.9-3.9, n= 4)
2.9
(2.0-2.3, n= 5)
2.0
(4.5-6.6, n= 6)
5.1
(2.0-2.3, n= 12)
2.1
(1.1-3.2, n= 12)
2.9
NC
NC
Nickel,
Diss.
NC
NC
NA
NA
(2.0-5.5, n= 9)
2.5
(3.6-9, n= 8)
6.5
(2.0-3.9, n= 13)
2.9
(3.3-4.8, n= 13)
3.9
(3.2-5.4, n= 6)
4.3
(5-9.4, n= 5)
6.3
(3.1-4.5, n= 5)
4.0
(7-12.5, n= 6)
8.7
(2.2-3.2, n= 12)
2.6
(3.4-8.4, n= 12)
4.8
NC
NC
Nickel,
Total
(13.5-16.0, n = 4)
14.6
(19.0-27.5, n = 3)
25.0
(11.0-13.1, n= 3)
11.0
NC
(9.4-28.6, n= 8)
12.5
(15.5-42.6, n= 8)
30.0
(6.7-49.0, n= 13)
12.5
(28.5-79.2, n= 14)
42.7
(7.8-54.0, n= 9)
19.0
(6.1-53.5, n= 8)
16.4
(20.1-33.2, n= 13)
22.6
(35.3-109.0, n= 13)
40.3
(10.7-24.3, n= 12)
19.8
(12.8-70, n= 12)
37.8
NC
NC
Zinc,
Diss.
(20.0-31.6, n = 7)
22.0
(45.0-51.0, n = 5)
50.0
(5.0-28.9, n= 9)
15.0
(43.9-120.8, n= 7)
51.0
(12.0-37.0, n= 33)
26.0
(43.9-78.1, n= 32)
51.8
(8.6-35.0, n= 19)
20.0
(51.8-106.0, n= 19)
86.0
(19.1-94.0, n= 13)
48.5
(21.5-225.3, n= 11)
125.0
(20.6-65.4, n= 19)
55.0
(43.8-244.3, n= 15)
149.5
(15.0-57.9, n= 13)
25.5
(53.0-245.0, n= 13)
100.5
(5.0-35.0, n= 5)
8.5
(51-68.5, n= 5)
68.0
Zinc,
Total
NA = Not available; studies containing 3 or more storms not available.
NC = Not calculated because fewer than 3 BMP studies for this category.
Interquartile Range = 25th percentile to 75th percentile values, calculated in Excel, which uses linear interpolation to calculate percentiles. For small sample sizes (particularly n<5),
interquartile values may vary depending on statistical package used.
(0.3-0.4, n =3)
0.3
NA
0.3
(0.1-0.5, n= 5)
0.5
(0.3-0.4, n= 3)
0.3
(0.2-2.5, n= 20)
0.4
(0.3-2.6, n= 20)
1.0
(0.1-0.7, n= 17)
0.2
(0.2-1.0, n= 17)
0.4
(0.2-0.6, n= 12)
0.4
(0.3-1.2, n= 11)
0.6
(0.2-0.4, n= 13)
0.3
(0.4-0.9, n= 14)
0.5
(0.1-0.2, n= 12)
0.2
(0.3-0.8, n= 12)
0.4
NC
NC
Cadmium,
Total
(0.3-0.5, n= 3)
NC
(0.2-0.2, n= 13)
0.2
(0.6-1.1, n= 12)
(0.2-0.2, n= 14)
1.1
1.1
(0.6-1.6, n= 12)
0.7
0.7
(0.2-0.4, n= 9)
0.3
(0.2-0.4, n= 8)
0.3
(0.1-0.2, n= 12)
0.2
(0.1-0.4, n= 13)
0.3
(0.1-0.2, n= 12)
0.1
(0.1-0.2, n= 12)
0.2
NC
NC
Cadmium,
Diss.
(0.5-1.1, n= 12)
1.7
(1.1-1.9, n= 6)
1.2
(0.9-1.2, n= 5)
2.1
(1.3-2.6, n= 6)
1.1
(0.9-1.2, n= 5)
(0.9-1.7, n= 8)
1.7
0.6
(0.6-1.2, n= 8)
(0.7-3.0, n= 12)
(0.5-2.4, n= 12)
1.2
2.0
1.2
(1.6-2.7, n= 9)
(0.9-2.3, n= 12)
(0.5-1.2, n= 12)
0.6
1.1
(0.5-2.2, n= 9)
NC
0.8
NC
NA
NA
Arsenic,
Total
Arsenic,
Diss.
Values below detection limits replaced with 1/2 of detection limit.
These descriptive statistics are based on different statistical measures than those used in the 2008 BMP Database tabular summary. Be aware that results will vary depending on whether a "BMP Weighted" (one median or average value represents each BMP) or "Storm
Weighted" (all storms for all BMPs included in statistical calculations) approach is used, as well as whether the median or another measure of central tendency is used. Several BMP Database publications in 2010 have focused on the storm-weighted approach, which may
result in some differences between this table and other published summaries.
Descriptive statistics calculated by weighting each BMP study equally. Each BMP study is represented by the median analyte value reported for all storms monitored at each BMP (i.e., "n" = number of BMP studies, as opposed to number of storm events). Depending on the
analysis objectives, researchers may also choose to use a storm-weighted analysis approach, a unit treatment process-based grouping of studies, or other screening based on design parameters and site-specific characteristics.
Analysis based on August 2010 BMP Database, which contains substantial changes relative to the 2008 BMP Database. Multiple BMPs have been recategorized into new BMP categories; therefore, the 2008 and 2010 data analysis are not directly comparable for several
BMP types.
This table contains descriptive statistics only. Values presented in this table should not be used to draw conclusions related to statistically significant differences in performance for BMP categories. (Hypothesis testing for BMP Categories is provided separately in other
BMP Database summaries available at www.bmpdatabase.org.)
**Permeable pavement data should be used with caution due to limited numbers of BMP studies and small numbers of storm events typically monitored at these sites. "Inflow" values are typically outflows monitored at a reference conventional paving site.
Table Notes:
Outflow
Table Key
Sample Type
Inflow
Permeable
Pavement**
Wetland Basin
Retention Pond
(aboveground
wet pond)
Media Filters
(various types)
Detention Basin
(aboveground
extended det.)
Grass Swale
Grass Buffer
Bioretention
(w/Underdrain)
BMP Category
Chapter 2
BMP Selection
2-7
BMP Selection
1.4
Chapter 2
Storage-Based Versus Conveyance-Based
BMPs in this manual generally fall into two categories: 1) storage-based and 2) conveyance-based.
Storage-based BMPs provide the WQCV and include bioretention/rain gardens, extended detention
basins, sand filters, constructed wetland ponds, retention ponds, and permeable pavement systems.
Conveyance-based BMPs include grass swales, grass buffers, constructed wetlands channels and other
BMPs that improve quality and reduce volume but only provide incidental storage. Conveyance-based
BMPs can be implemented to help achieve objectives in Step 1 of the Four Step Process. Although
conveyance BMPs do not satisfy Step 2 (providing the WQCV), they can reduce the volume requirements
of Step 2. Storage-based BMPs are critical for Step 2 of the Four Step Process. Site plans that use a
combination of conveyance-based and storage-based BMPs can be used to better mimic pre-development
hydrology.
1.5
Volume Reduction
BMPs that promote infiltration or that incorporate
Hydromodification
evapotranspiration have the potential to reduce the
volume of runoff generated. Volume reduction is a
The term hydromodification refers to altered
fundamental objective of LID. Volume reduction has
hydrology due to increased imperviousness
many benefits, both in terms of hydrology and
combined with constructed of conveyance
pollution control. While stormwater regulations have
systems (e.g., pipes) that convey stormwater
traditionally focused on runoff peak flow rates,
efficiently to receiving waters.
emerging stormwater regulations require BMPs to
Hydromodification produces increased peaks,
mimic the pre-development hydrologic budget to
volume, frequency, and duration of flows, all of
minimize effects of hydromodification. From a
which can result in stream degradation,
pollution perspective, decreased runoff volume
including stream bed down cutting, bank
translates to decreased pollutant loads. Volume
erosion, enlarged channels, and disconnection
reduction has economic benefits, including potential
of streams from the floodplain. These factors
reductions in storage requirements for minor and
lead to loss of stream and riparian habitat,
major events, reduced extent and sizing of
reduced aquatic diversity, and can adversely
conveyance infrastructure, and cost reductions
impact the beneficial uses of our waterways.
associated with addressing channel stability issues.
UDFCD has developed a computational method for
quantifying volume reduction. This is discussed in detail in Chapter 3.
Infiltration-based BMPs can be designed with or without underdrains, depending on soil permeability and
other site conditions. The most substantial volume reductions are generally associated with BMPs that
have permeable sub-soils and allow infiltration to deeper soil strata and eventually groundwater. For
BMPs that have underdrains, there is still potential for volume reduction although to a lesser degree. As
runoff infiltrates through BMP soils to the underdrain, moisture is retained by soils. The moisture
eventually evaporates, or is taken up by vegetation, resulting in volume reduction. Runoff that drains
from these soils via gravity to the underdrain system behaves like interflow from a hydrologic perspective
with a delayed response that reduces peak rates. Although the runoff collected in the underdrain system is
ultimately discharged to the surface, on the time scale of a storm event, there are volume reduction
benefits.
Although effects of evapotranspiration are inconsequential on the time scale of a storm event, on an
annual basis, volume reduction due to evapotranspiration for vegetated BMPs such as retention and
constructed wetland ponds can be an important component of the hydrologic budget. Between events,
evapotranspiration lowers soil moisture content and permanent pool storage, providing additional storage
capacity for subsequent events.
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BMP Selection
Other surface BMPs also provide volume reduction through a combination of infiltration, use by the
vegetation and evaporation. Volume reduction provided by a particular BMP type will be influenced by
site-specific conditions and BMP design features. National research is ongoing with regard to estimating
volume reduction provided by various BMP types. Based on analysis of BMP studies contained in the
International Stormwater BMP Database, Geosyntec and WWE (2010) reported that normally-dry
vegetated BMPs (filter strips, vegetated swales, bioretention, and grass lined detention basins) appear to
have substantial potential for volume reduction on a long-term basis, on the order of 30 percent for filter
strips and grass-lined detention basins, 40 percent for grass swales, and greater than 50 percent for
bioretention with underdrains. Bioretention facilities without underdrains would be expected to provide
greater volume reduction.
1.6
Pretreatment
Design criteria in this manual recommend forebays for extended detention basins, constructed wetland
basins, and retention ponds. The purpose of forebays is to settle out coarse sediment prior to reaching the
main body of the facility. During construction, source control including good housekeeping can be more
effective than pre-treatment. It is extremely important that high sediment loading be controlled for BMPs
that rely on infiltration, including permeable pavement systems, rain gardens, and sand filter extended
detention basins. These facilities should not be brought on-line until the end of the construction phase
when the tributary drainage area has been stabilized with permanent surfaces and landscaping.
1.7
Treatment Train
The term "treatment train" refers to multiple BMPs in series (e.g., a disconnected roof downspout
draining to a grass swale draining to a constructed wetland basin.) Engineering research over the past
decade has demonstrated that treatment trains are one of the most effective methods for management of
stormwater quality (WERF 2004). Advantages of treatment trains include:

Multiple processes for pollutant removal: There is no "silver bullet" for a BMP that will address
all pollutants of concern as a stand-alone practice. Treatment trains that link together complementary
processes expand the range of pollutants that can be treated with a water quality system and increase
the overall efficiency of the system for pollutant removal.

Redundancy: Given the natural variability of the volume, rate and quality of stormwater runoff and
the variability in BMP performance, using multiple practices in a treatment train can provide more
consistent treatment of runoff than a single practice and provide redundancy in the event that one
component of a treatment train is not functioning as intended.

Maintenance: BMPs that remove trash, debris, coarse sediments and other gross solids are a
common first stage of a treatment train. From a maintenance perspective, this is advantageous since
this first stage creates a well-defined, relatively small area that can be cleaned out routinely.
Downgradient components of the treatment train can be maintained less frequently and will benefit
from reduced potential for clogging and accumulation of trash and debris.
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BMP Selection
1.8
Chapter 2
Online Versus Offline Facility Locations
The location of WQCV facilities within a development site and watershed requires thought and planning.
Ideally this decision-making occurs during a master planning process. Outfall system plans and other
reports may depict a recommended approach for implementing WQCV on a watershed basis. Such
reports may call for a few large regional WQCV facilities, smaller sub-regional facilities, or an onsite
approach. Early in the development process, it is important to determine if a master planning study has
been completed that addresses water quality and to attempt to follow the plan's recommendations.
When a master plan identifying the type and location of water quality facilities has not been completed, a
key decision involves whether to locate a BMP online or offline. Online refers to locating a BMP such
that all of the runoff from the upstream watershed is intercepted and treated by the BMP. A single online
BMP should be designed to treat both site runoff and upstream
(offsite) runoff. Locating BMPs offline requires that all onsite
catchment areas flow though a BMP prior to combining with
flows from the upstream (offsite) watershed. Be aware, when
When water quality BMPs
water quality BMPs are constructed in "Waters of the State"
are constructed in "Waters
they must be accompanied by upstream treatment controls and
of the State," they must be
source controls.
accompanied by upstream
Online WQCV facilities are only recommended if the offsite
treatment and source
watershed has less impervious area than that of the onsite
controls.
watershed. Nevertheless, online WQCV facilities must be
sized to serve the entire upstream watershed based on future
development conditions. This recommendation is true even if
upstream developments have installed their own onsite WQCV
facilities. The only exception to this criterion is when multiple
online regional or sub-regional BMPs are constructed in series
and a detailed hydrologic model is prepared to show appropriate sizing of each BMP. The maximum
watershed recommended for a water quality facility is approximately one square mile. Larger watersheds
can be associated with decreased water quality.
1.9
Integration with Flood Control
In addition to water quality, most projects will require detention for flood control, whether on-site, or in a
sub-regional or regional facility. In many cases, it is efficient to combine facilities since the land
requirements for a combined facility are lower than for two separate facilities. Wherever possible, it is
recommended WQCV facilities be incorporated into flood control detention facilities.
Local jurisdictions in the Denver area use different approaches for sizing volumes within a combined
water quality and quantity detention facility. This varies from requiring no more than the 100-year
detention volume, even though the WQCV is incorporated within it, to requiring the 100-year detention
volume plus the full WQCV. This manual does not stipulate or recommend which policy should be used.
When a local policy has not been established, UDFCD suggests the following approach:

Water Quality: The full WQCV is to be provided according to the design procedures documented in
this manual.

Minor Storm (not EURV): The full WQCV, plus the full minor storm detention volume, is to be
provided.
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BMP Selection

100-Year Storm: One-half the WQCV plus the full 100-year storm event volume should be provided
for volumes obtained using the empirical equations or the FAA Method. When the analysis is done
using hydrograph routing methods, each level of controls needs to be accounted for and the resultant
100-year control volume used in final design.

100-Year Storm using Full Spectrum Detention: The full 100-year storm event volume should be
provided according to the design protocol provided in the Storage chapter of Volume 2.
The Storage chapter in Volume 2 provides design criteria for full spectrum detention, which shows more
promise in controlling the peak flow rates in receiving waterways than the multi-stage designs described
above. Full spectrum detention not only addresses the WQCV for controlling water quality and runoff
from frequently occurring runoff events, but also extends that control for all return periods through the
100-year event and closely matches historic peak flows downstream.
Finally, designers should also be aware that water quality BMPs, especially those that promote
infiltration, could result in volume reductions for flood storage. These volume reductions are most
pronounced for frequently occurring events, but even in the major event, some reduction in detention
storage volume can be achieved if volume-reduction BMPs are widely used on a site. Additional
discussion on volume reduction benefits, including a methodology for quantifying effects on detention
storage volumes, is provided in Chapter 3.
1.9.1
Sedimentation BMPs
Combination outlets are relatively straightforward for most BMPs in this manual. For BMPs that utilize
sedimentation (e.g. EDBs, constructed wetland ponds, and retention ponds) see BMP Fact Sheet T-12.
This Fact Sheet shows examples and details for combined quality/quantity outlet structures.
1.9.2
Infiltration/Filtration BMPs
For other types of BMPs (e.g. rain gardens, sand filters, permeable pavement systems, and other BMPs
utilizing processes other than sedimentation), design of a combination outlet structure generally consists
of multiple orifices to provide controlled release of WQCV as well as the minor and major storm event.
Incorporation of full spectrum detention into these structures requires reservoir routing. The UDDetention worksheet available at www.udfcd.org can be used for this design. When incorporating flood
control into permeable pavement systems, the design can be simplified when a near 0% slope on the
pavement surface can be achieved. The flatter the pavement the fewer structures required. This includes
lateral barriers as well as outlet controls since each pavement cell typically requires its own outlet
structure. When incorporating flood control into a rain garden, the flood control volume can be placed on
top of or downstream of the rain garden. Locating the flood control volume downstream can reduce the
total depth of the rain garden, which will result in a more attractive BMP, and also benefit the vegetation
in the flood control area because inundation and associated sedimentation will be less frequent, limited to
events exceeding the WQCV.
1.10
Land Use, Compatibility with Surroundings, and Safety
Stormwater quality areas can add interest and diversity to a site, serving multiple purposes in addition to
providing water quality functions. Gardens, plazas, rooftops, and even parking lots can become amenities
and provide visual interest while performing stormwater quality functions and reinforcing urban design
goals for the neighborhood and community. The integration of BMPs and associated landforms, walls,
landscape, and materials can reflect the standards and patterns of a neighborhood and help to create lively,
safe, and pedestrian-oriented districts. The quality and appearance of stormwater quality facilities should
August 2011
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BMP Selection
Chapter 2
reflect the surrounding land use type, the immediate context, and the proximity of the site to important
civic spaces. Aesthetics will be a more critical factor in highly visible urban commercial and office areas
than at a heavy industrial site. The standard of design and construction should maintain and enhance
property values without compromising function (WWE et al. 2004).
Public access to BMPs should be considered from a safety perspective. The highest priority of engineers
and public officials is to protect public health, safety, and welfare. Stormwater quality facilities must be
designed and maintained in a manner that does not pose health or safety hazards to the public. As an
example, steeply sloped and/or walled ponds should be avoided. Where this is not possible, emergency
egress, lighting and other safety considerations should be incorporated. Facilities should be designed to
reduce the likelihood and extent of shallow standing water that can result in mosquito breeding, which
can be a nuisance and a public health concern (e.g., West Nile virus). The potential for nuisances, odors
and prolonged soggy conditions should be evaluated for BMPs, especially in areas with high pedestrian
traffic or visibility.
1.11
Maintenance and Sustainability
Maintenance should be considered early in the planning and design phase. Even when BMPs are
thoughtfully designed and properly installed, they can become eyesores, breed mosquitoes, and cease to
function if not properly maintained. BMPs can be more effectively maintained when they are designed to
allow easy access for inspection and maintenance and to take into consideration factors such as property
ownership, easements, visibility from easily accessible points, slope, vehicle access, and other factors.
For example, fully consider how and with what equipment BMPs will be maintained in the future. Clear,
legally-binding written agreements assigning maintenance responsibilities and committing adequate funds
for maintenance are also critical (WWE et al. 2004). The MS4 permit holder may also require right of
access to perform emergency repairs/maintenance should it become necessary.
Sustainability of BMPs is based on a variety of considerations related to how the BMP will perform over
time. For example, vegetation choices for BMPs determine the extent of supplemental irrigation required.
Choosing native or drought-tolerant plants and seed mixes (as recommended in the Revegetation chapter
of Volume 2) helps to minimize irrigation requirements following plant establishment. Other
sustainability considerations include watershed conditions. For example, in watersheds with ongoing
development, clogging of infiltration BMPs is a concern. In such cases, a decision must be made
regarding either how to protect and maintain infiltration BMPs, or whether to allow use of infiltration
practices under these conditions.
1.12
Costs
Costs are a fundamental consideration for BMP selection, but often the evaluation of costs during
planning and design phases of a project focuses narrowly on up-front, capital costs. A more holistic
evaluation of life-cycle costs including operation, maintenance and rehabilitation is prudent and is
discussed in greater detail in Section 4 of this chapter. From a municipal perspective, cost considerations
are even broader, involving costs associated with off-site infrastructure, channel stabilization and/or
rehabilitation, and protection of community resources from effects of runoff from urban areas.
2-12
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Chapter 2
2.0
BMP Selection
BMP Selection Tool
To aid in selection of BMPs, UDFCD has developed a BMP selection tool (UD-BMP) to guide users of
this manual through many of the considerations identified above and to determine what types of BMPs
are most appropriate for a site. This tool helps to screen BMPs at the planning stages of development and
can be used in conjunction with the BMP-REALCOST tool described in Section 4. Simplified schematics
of the factors considered in the UD-BMP tool are provided in Figures 2-1, 2-2, and 2-3, which correspond
to highly urbanized settings, conventional developments, and linear construction in urbanized areas.
Separate figures are provided because each setting or type of development presents unique constraints.
Highly urbanized sites are often lot-line to lot-line developments or redevelopments with greater than 90
percent imperviousness with little room for BMPs. Linear construction typically refers to road and rail
construction.
Figure 2-1. BMP Decision Tree for Highly Urbanized Sites
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Figure 2-2. BMP Decision Tree for Conventional Development Sites
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Figure 2-3. BMP Decision Tree for Linear Construction in Urbanized Areas
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Life Cycle Cost and BMP Performance Tool
The importance of cost effective BMP planning and selection is gaining recognition as agencies
responsible for stormwater management programs continue to face stricter regulations and leaner budgets.
The goal of the BMP-REALCOST tool is to help select BMPs that meet the project objectives at the
lowest unit cost, where the project objectives are quantifiable measures such as reducing pollutant loads
or runoff volumes to a receiving water. To do so, UDFCD has developed an approach that provides
estimates for both the whole life costs and performance of BMPs. The approach was developed to be
most effective at the large-scale, planning phase; however, it can also be applied to smaller scales during
the design phase, perhaps with minor loss of accuracy. The BMP-REALCOST spreadsheet tool
incorporates this approach and requires minimal user inputs in order to enhance its applicability to
planning level evaluations. An overview of the general concepts providing the underlying basis of the
tool follows.
3.1
BMP Whole Life Costs
Whole life costs (also known as life cycle costs) refer to all costs that occur during the economic life of a
project. This method of cost estimating has gained popularity in the construction and engineering fields
over the past few decades and the American Society of Civil Engineers (ASCE) encourages its use for all
civil engineering projects. Generally, the components of the whole life cost for a constructed facility
include construction, engineering and permitting, contingency, land acquisition, routine operation and
maintenance, and major rehabilitation costs minus salvage value. In addition, UDFCD recommends the
cost of administering a stormwater management program also be included as a long-term cost for BMPs.
Reporting whole life costs in terms of net present value (NPV) is an effective method for comparing
mutually exclusive alternatives (Newnan 1996).
To understand the value of using whole life cost estimating, one must first realize how the various costs
of projects are generally divided amongst several stakeholders. For example, a developer is typically
responsible for paying for the "up front" costs of construction, design, and land acquisition; while a
homeowners' association or stormwater management agency becomes responsible for all costs that occur
after construction. Many times, the ratios of these costs are skewed one way or another, with BMPs that
are less expensive to design and construct having greater long-term costs, and vice versa. This promotes
a bias, depending on who is evaluating the BMP cost effectiveness. Whole life cost estimating removes
this bias; however, successful implementation of the concept requires a cost-sharing approach where the
whole life costs are equitably divided amongst all stakeholders.
The methods incorporated into the BMP-REALCOST tool for estimating whole life costs are briefly
described below. All cost estimates are considered "order-of-magnitude" approximations, hence
UDFCD's recommendation of using this concept primarily at the planning level.

Construction Costs: Construction costs are estimated using a parametric equation that relates costs
to a physical parameter of a BMP; total storage volume (for storage-based BMPs), peak flow capacity
(for flow-based or conveyance BMPs) or surface area (for permeable pavements).

Contingency/Engineering/Administration Costs: The additional costs of designing and permitting
a new BMP are estimated as a percentage of the total construction costs. For Denver-area projects, a
value of 40% is recommended if no other information is available.

Land Costs: The cost of purchasing land for a BMP is estimated using a derived equation that
incorporates the number of impervious acres draining to the BMP and the land use designation in
which the BMP will be constructed.
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
Maintenance Costs: Maintenance costs are estimated using a derived equation that relates average
annual costs to a physical parameter of the BMP.

Administration Costs: The costs of administering a stormwater management program are estimated
as percentage of the average annual maintenance costs of a BMP. For Denver-area projects, a value
of 12% is recommended if no other information is available.

Rehabilitation/Replacement Costs: After some period of time in operation, a BMP will require
"major" rehabilitation. The costs of these activities (including any salvage costs or value) are
estimated as a percentage of the original construction costs and applied near the end of the facility's
design life. The percentages and design lives vary according to BMP.
3.2
BMP Performance
The performance of structural BMPs can be measured as the reduction in stormwater pollutant loading,
runoff volume and runoff peak flows to the receiving water. It is generally acknowledged that estimating
BMP performance on a storm-by-storm basis is unreliable, given the inherent variability of stormwater
hydrologic and pollutant build-up/wash off processes. Even if the methods to predict event-based BMP
performance were available, the data and computing requirements to do so would likely not be feasible at
the planning level. Instead, UDFCD recommends an approach that is expected to predict long-term (i.e.
average annual) BMP pollutant removal and runoff volume reduction with reasonable accuracy, using
BMP performance data reported in the International Stormwater BMP Database (as discussed in
Section 1.3).
3.3
Cost Effectiveness
The primary outputs of the BMP-REALCOST tool include net present value (NPV) of the whole life costs
of the BMP(s) implemented, the average annual mass of pollutant removed (PR, lbs/year) and the average
annual volume of surface runoff reduced (RR, ft3/year). These reported values can then be used to
compute a unit cost per lb of pollutant (CP) or cubic feet of runoff (CR) removed over the economic life
(n, years) of the BMP using Equations 2-1 and 2-2, respectively.
𝐶𝑃 =
𝐶𝑅 =
4.0
𝑁𝑃𝑉
𝑛𝑃𝑅
Equation 2-1
𝑁𝑃𝑉
𝑛𝑅𝑅
Equation 2-2
Conclusion
A variety of factors should be considered when selecting stormwater management approaches for
developments. When these factors are considered early in the design process, significant opportunities
exist to tailor stormwater management approaches to site conditions. Two worksheets are available at
www.udfcd.org for the purpose of aiding in the owner or engineer in the proper selection of treatment
BMPs. The UD-BMP tool provides a list of BMPs for consideration based on site-specific conditions.
BMP-REALCOST provides a comparison of whole life cycle costs associated with various BMPs based
on land use, watershed size, imperviousness, and other factors.
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References
Geosyntec and Wright Water Engineers, 2010. International Stormwater Best Management Practices
(BMP) Database Technical Summary: Volume Reduction. Prepared for the Water Environment
Research Foundation, Federal Highway Administration, and the Environment and Water
Resources Institute of the American Society of Civil Engineers.
Hartigan, J.P. 1989. Basis for Design of Wet Detention Basin BMPs. Design of Urban Runoff Quality
Controls. Proceedings Engineering Foundation Conference. American Society of Civil
Engineers (ASCE): New York, NY.
Metcalf and Eddy, Inc. 2003. Wastewater Engineering, Treatment, Disposal and Reuse. Fourth Edition.
Revised by G. Tchobanoglous and F.L. Burton. McGraw Hill: New York, NY.
Newnan, Donald G. 1996. Engineering Economic Analysis. Sixth Edition. Engineering Press: San
Jose, CA.
Randall, C.W., K. Ellis, T.J. Grizzard, and W.R. Knocke. 1982. Urban Runoff Pollutant Removal by
Sedimentation. Stormwater Detention Facilities. Proceedings of the Engineering Foundation
Conference. ASCE: New York, NY.
Roesner, L. and C. Olson. 2009. BMP-REALCOST.xls Spreadsheet Tool. Prepared for Urban Drainage
and Flood Control District: Denver, CO.
Stahre, P. and B. Urbonas. 1990. Stormwater Detention for Drainage, Water Quality, and CSO
Management. Prentice Hall.
Water Environment Federation (WERF). 2005. Critical Assessment of Stormwater Treatment Controls
and Control Selection Issues. 02-SW-01. WERF: Alexandria, VA: IWA Publishing: London.
Wright Water Engineers, Inc., Wenk Associates, Muller Engineering Company, Inc., Matrix Design
Group, and Smith Environmental. 2004. City and County of Denver Water Quality Management
Plan. Denver, CO
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